Apparatus for generating sound



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United States Patent O APPARATUS FOR GE NERATING SOUND Stanley F. White, Chicago, Ill., assignor, by mesne as signments, to Edward V. Bracken, Darien, Conn.

Application November 24, 1952, Serial No. 322,243

7 Claims. (Cl. 181-31) This invention relates to methods and apparatus for generating sounds and more particularly to sound generation in partially or totally enclosed spaces such as rooms or auditoriums.

In attempting to generate or reproduce sounds uniformly over the audible range of frequencies, great difficulties have heretofore been experienced with generation of sounds in the low frequency end of the audible range. In particular, according to well developed free field theory and practice, the sound generator must have a certain effective area in relation to the wavelength being produced and for frequencies below 400 cycles per second, the sound generator, whether it is a diaphragm alone or a diaphragm coupled to a horn, must have an extremely large effective area. This rigid requirement is, in general, true in systems designed for outdoor use and no solution is herein proposed to the problems involved with such outdoor systems.

This invention is concerned primarily with sound generation indoors in partially or totally enclosed spaces although certain features of the invention may, as well, have application in outdoor systems. In all cases in which any scientific analysis has been applied to sound generators for indoor use of which I am aware, an attempt has been made to solve the low frequency problems either by the free field theory or by the provision of resonant or sound absorption devices which inherently produce distortions and non-uniformities in the system. From all appearances, the fact that an enclosed space does not behave acoustically according to free field practice has, if considered at all, been considered an unfortunate circumstance of nature and as a problem that architects should solve by making all rooms behave acoustcally according to free field theory. From a direct approach to the problem, it has been found that the low frequency characteristics of enclosed spaces are a great advantage, rather than a disadvantage, and that sound generators effective at the low frequency end of the audible range and uniformly effective over the entire audible range can be much more efficient while of considerably less size.

Where the wavelength of sound is large compared to the dimensions of an enclosed space, the space acts as an acoustical compliance determined by factors such as the volume of the space, the velocity of sound, the effective area of the diaphragm or other sound generator and an air constant. With speaker diaphragms or cones of conventional sizes, under l inches in diameter, some dimculty is experienced in efiicicntly generating sounds at the low frequency end of the audible range, under 200 cycles per second, in enclosed spaces not because of the low air mass reactance and resistance as is the case in free field practice but because the movement or excursion of the diaphragm must be very large if appreciable pressure changes within the enclosed space are to take place. For example, a inch speaker in an infinite baffle will begin to drop 0E in efficiency below 400 2,866,513 Patented Dec. 30, 1958 ICC cycles because of the limit of excursion of the speaker diaphragm. By this invention, an acoustical connector or coupler is provided between one face of a speaker diaphragm and the enclosed space so as to efficiently couple the diaphragm to the enclosed space at the low frequency end of the audible range and secure large pressure changes with only limited movement or excursion of the speaker diaphragm.

This connector has a throat coupled to the speaker diaphragm and a mouth opening into the enclosed space with the cross-sectional arca of the sound path continuously increasing or flaring out from the throat to the mouth and preferably increasing according to an exponential function of the distance from the throat.

While this connector has some similarity to the expo nential horns used in systems constructed according to free field theory, it is to be noted that it is, in operation quite different. In particular, the lowest frequency that can be transmitted by a horn is proportional to the rate of flare thereof and, for a given mouth area, a horn must have a length increasing as the rate of flare decreases. Accordingly, to effectively transmit very low frequencies a horn constructed according to free field theory must be extremely long. Where size has been a limitation as with horns constructed for home use, it has heretofore been the practice, under free field theory, to have a mini mum flare cut-off of about 1GO cycles per second.

With a connector constructed according to this invention, on the other hand, it has been found that a lengthy connector is not necessary or even desirable and that extremely efficient generation of low frequencies can be achieved in enclosed spaces with a connector having a low flare cut-off frequency and having, at the same time, a very short length. In particular, very excellent results have been achieved with a flare cut-off of less than 50 cycles and with the cross-sectional area of the connector doubling three times or less from the throat to the mouth, the area of the mouth, of course, being substantially larger than the area of the diaphragm. Under free field theory, this would not be considered possible.

Another important feature of the present invention is in the provision of a chamber serving the dual function of coupling the speaker diaphragm to the throat of the connector at low frequencies and acting to present a non-resonant compliance to the diaphragm up to the high frequency limit of sound transmission from the diaphragm. By this feature, the mean distance from the diaphragm to the facing surface of the chamber is not greater than one-half wavelength at the highest frequency of transmission and is preferably one-fourth wavelength or less at that frequency.

A further feature of the invention is in the construction of the throat between the speaker chamber and the connector which provides a smooth transition between the low frequency range where the diaphragm is coupled to the connector and the higher frequency range where the chamber acts only as a non-resonant compliance as the frequency goes up, the impedance at the throat of the connector goes up and the impedance of the chamber goes down so that the transition is effected and by proper dimensioning of the throat, the efficiency may be made uniform below, through and above the transition range of frequencies. In certain cases, an intermediate high rate of flare connector section between the chamber and the main connector achieves improved results and a smooth transition but such an intermediate section is not required in all cases.

A still further feature of the invention is in simple readily constructed cabinets embodying the highly advantageous connector, chamber and throat features described above by which a minimum number of bafiies are disposed in a manner such as to efiiciently utilize all the space within the cabinet, provide improved support of the cabinet walls and minimize a possibility of reverberations and resonances.

Still another feature is in the provision of a speaker with a large diaphragm and a small diaphragm within the large diaphragm and in the construction of the speaker and the disposition of the speaker within the chamber in a manner such that both the large diaphragm and the small diaphragm transmit sound into the chamber up to a frequency below which the chamber becomes resonant with the small diaphragm only being effective at frequencies above that frequency and with means preventing transmission from the small diaphragm into the chamber above that frequency.

Yet another feature of the invention is in the matching of a speaker to the connector in a manner such as to obtain uniform generation of sound at the low frequency end of the audible range. By this feature, the impedance characteristics of the connector are obtained through measurement in free air, in a free field sound room or according to free field theory, and the connector will have a first resonant peak at a frequency which is somewhat above the are cut-off frequency. It has been found that by making the resonant frequency of the speaker in free air equal the frequency of the first resonant peak of the connector in free air, optimum results are obtained with a uniform generation of sound from the lowest frequency transmitted.

An object of this invention, accordingly, is to provide improved means and methods for generation of sound in partially or totally enclosed spaces.

Anther object of this invention is to provide an improved, simple, readily constructed, eicient and compact sound generating enclosure.

Another object of this invention is to provide an immeans and methods for obtaining uniform generation of sound in the audible range.

This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with 'the accompanying drawings which illustrate preferred embodiments and in which:

Figure 1 is a perspective view looking into one preferred sound generating enclosure of this invention with one side removed;

Figure 2 is a sectional view taken substantially along line II-II of Figure l;

Figure 3 is a sectional view taken substantially along line III-lll in Figure l;

Figure 4 is a sectional view taken substantially along line lV-IV in Figure 2;

Figure 5 is a circuit diagram of a network useful in this invention;

Figure 6 is a cross-sectional view of another preferred form of sound generating enclosure according to this invention;

Figure 7 is a cross-sectional view of still another preferred form of sound generating enclosure; and

Figure 8 is a cross-sectional view taken substantially along line VIII- VIII of Figure 7.

Reference numeral 10 generally designates a preferred form of sound generator constructed according to this invention which includes a box-like rectangular cabinet having a front 11, a back l2, a top 13, a bottom 14 and a pair of sides 15 only one of which is shown, the other being removed to illustrate the internal construction, in Figure l.

The front 11 has an opening 16, preferably circular, with a speaker 17 mounted behind the opening 16 and having a conical diaphragm or cone 18 with an electromechanical transducer for driving the cone 18. The construction of the speaker 17 will be described in more detail hereinafter.

The sound generating enclosure 10 is particularly constructed and adapted for generating sounds indoors in 4 partially or totally enclosed spaces. As has been pointed out above, a speaker of conventional size, less than l5 inches in diameter, cannot efficiently generate sounds at the low frequency end of the audible range, even when mounted in a very large baie, due to the limited excursion of the speaker cone. According to this invention, an acoustical connector is provided between the back face of the diaphragm and the enclosed space in which the generator 10 operates to couple the diaphragm 18 to the enclosed space and produce large pressure variations therein with small excursion of the diaphragm 18.

This acoustical connector has a mouth defined by a rectangular opening at the lower end of the cabinet front, the front 11 having a lower edge 19 spaced upwardly from the bottom 14 for this purpose. A baliie 20 is secured between the sides 15 and extends rearwardly from the lower edge 19 of the front 11 part way toward the back 12 and a second bafiie 21 is also secured between the sides 15 and extends upwardly from the rear end of the bafe to an upper edge 22 spaced from the cabinet top 13. The baliies 2|) and 21 are inclined relative the bottom 14 and the back 12 so as to provide a sound path from the throat defined by upper edge 22 of the baie 21 and the top 13 to the mouth below lower edge 19 of the front 11 which has a continuously increasing cross-sectional area.

The acoustical connector may preferably include a further extended sound path between the diaphragm 18 and the throat defined by the upper edge 22 of the baffle 21 and the cabinet top 13. For this purpose, a third baie 23 is disposed between the baflie 21 and the front 11 and extends downwardly from the top 13 to a lower edge 24 spaced above the baie 20. Additional baffles 25 and 26 are provided between the bafiie 23 and the front 11, the bafiie 25 extending downwardly from the top 13 to a lower edge 27 and the baffie 26 extending upwardly from the bailie 20 to an upper edge 28. As shown in Figure 2, the lower edge 27 of the baffle 25 and the upper edge 28 of the bafe 26 are centrally recessed at 29 and 30 to receive a rear portion of the speaker 17, the remaining portions of the edges 27 and 28 being in spaced parallel relation to define a pair of throats 31 and 32 on opposite sides of the speaker 17. A pair of boards 33 and 34 which have curved lower surfaces are secured together and atiixed between the baffie 25 adjacent the lower edge 27 thereof on either side of the speaker 17 and a recess 35 in the front face of the baie 23. Disposed between the bafiies 23 and 25 and over the rear portion of the speaker 17 are boards 36, 37 and 38 which prevent transmission of sound into the space thereabove.

To provide a sound path of continuously increasing area from each of the throats 31 and 32 toward the mouth of the acoustical connector, a baffle member 39 is disposed between the baffle 23 and the baie 26, and has opposite inclined side surfaces 40 and 41 which cooperate with the facing surfaces of the cabinet sides 15.

Optimum results are obtained when the acoustical connector has a cross-sectional area continuously increas ing according to an exponential function of the distance from the throat thereof. It is also desirable to eliminate. insofar as possible, any parallel surfaces which might permit a reverberant effect therebetween. For thcsc reasons, additional baffles are preferably provided. These additional baies herein include a set of four boards "l2,

43, 44 and 45 which have concave generally cylindrical upper surfaces and are affixed together and in reces-1e; 46, 47 and 4S in the bafes 21, 20 and 26, respectively. to form a concave cylindrical surface facing the lower edge 24 of the baftie 23 and forming a smooth transition from the rear surface of the baie 26 to the front surface of the baffle 21. Another baliie 49 of semi-cylindrical shape is affixed in recesses 50, 51 and 52 in the back 12, top 13 and batiie 23, respectively, and has a concave generally cylindrical surface facing the upper edge 22 of the baliie 2l and forming a smooth transition from the rearsurface of the baille 23 to the front surface of the back 12. A further baille 53 is aixed in recesses 54 and S of the back 12 and bottom 14, respectively, and has a concave surface extending from the front surface of the back 12 to the top surface of the bottom 14.

It will be appreciated that the paths from the throats 31 and 32 to the mouth of the acoustical connector may thus have cross-sectional areas which are very nearly a true exponential function of the distance from the throats. It will also be apparent that the angular disposition of the baffle defined by the boards 42, 4.1, 4d and 45, the baille 49 and the baffle 53 prevents reverberation between parallel surfaces. In this connection, it may also be noted that at the turn in the sound path which is nearest the mouth of the` acoustical connector and therefore the turn at which the cross-sectional arca of the sound path is greatest, the angle of the turn is 90 so that there are no parallel walls which might otherwise cause reverberations.

In the sound generator 10, the acoustical connector is dimensioned to take advantage of the acoustical compliance of large rooms and auditoriums as well as ordinary rooms. [t has a total throat area of approximately 45 square inches and a total mouth area of about 264 square inches with the area doubling less than two and one-half times from the throat to the mouth. The total length of the connector is approximately 84 inches and has a flare cut-off frequency of less than 30 cycles per second. While the area of the connector mouth might be greatly increased, the length of the connector greatly increased and the flare cut-ott frequency increased to obtain slightly improved results at certain frequencies, it has been found that there is no necessity, in practice, for a are cut-off frequency higher than 6l) cycles per second and there is no perceptible advantage obtained by making the connector mouth greater than ve square feet in area or by making the connector of a length such that the area is doubled more than three times from the throat to the mouth. Such would, in fact, be highly impractical since the volume of the generator would be increased and it would be proportionately more expensive to manufacture.

A highly important feature of this invention is in the chamber behind the diaphragm 18 which serves the dual function of coupling the diaphragm to the twin throats 31 and 32 of the acoustical connector above described at lower frequencies and acts as a non-resonant compliance to the highest frequency transmitted by the diaphragm 18. This chamber is defined by the cabinet front 11 and the aligned baffles 25 and 26. The mean distance between the front 11 and the baffles 25 and 26 should be not greater than one-half wavelength at the highest frequency transmitted by the diaphragm 18 and is preferably one-fourth wavelength or less. In the sound generator 10, the diaphragm 18 is so excited as to not transmit any appreciable sound above a frequency of about 120() cycles per second at which the wavelength is approximately one foot. Accordingly, the spacing be tween the battles 25 and 26 and the front 11 should be less than six inches and preferably this spacing is less than three inches. This close spacing not only prevents the possibility of resonances at the higher frequencies but also results in a much more effective coupling between the diaphragm 18 and the throats 31 and 32 of the connector since at low frequencies, the sound paths from all parts of the diaphragm to the throats is of generally uniform length and there is very little phase shift at the throats between sounds generated by separated parts of the diaphragm 18. Another advantage in the close spacing is in the fact that the diaphragm is disposed between the facing surfaces of the sides and the facing surfaces of the top 13 and the battle Z so that the possibility of reverberation between these surfaces is minimized. In this connection, a specific feature of the invention is in the provision of baffle boards in the chamber to eliminate 6 the possibility of such reverberations. As shown in Figures l and 2, battle boards 56, 57, 58, 59 and 60 are affixed between the front cabinet wall 11 and the battles 25 and 26 with the boards 56, 57, 58 and 60 inclined so as to eliminate any facing surfaces not having the diaphragm 18 therebetween. These battles 56-60 not only prevent the possibility of reverberations within the chamber at the higher frequencies but also provide improved coupling between the diaphragm 18 and the throats 31 and 32 at low frequencies since the sound from all parts of the diaphragm is directed toward the throats.

In operation, the impedance at the throats 31 and 32 increases with increased frequency while the reactance of the chamber behind the diaphragm 18 decreases with increased frequency so that at a certain intermediate range of frequencies, the transmission of sounds through the acoustical connector is stopped and above that range only the chamber is effective. The center frequency in that intermediate range, that is, the cross-over frequency, is preferably about cycles per second although the cross-over may be at any desired frequency. This frequency is determined by the characteristics of the enclosed space in which the sound generator is operating, the characteristics of the acoustical connector and the characteristics of the chamber behind the diaphragm. The acoustical structure at the throat of the connector is also highly important and, as will be pointed out hereinafter in connection with other preferred embodiments of the invention the coupling between the chamber and the throat of the connector may be adjusted so as to obtain a smooth cross-over.

Another important feature of the invention is in a special construction of the speaker 17 and in a particu lar structural relation between the speaker and the cabinet. Referring now to Figure 4, the speaker 17 includes, in addition to the large diaphragm 18, a small diaphragm 61 arranged to transmit sounds at frequencies above the frequency at which the large diaphragm 18 is effective. This small diaphragm 61 may be of generally segmental spherical shape and is affixed within a central opening of the large diaphragm 18. The speaker 17 includes a frame which includes a large outer ring portion (not shown) affixed to the front cabinet wall 11 and supporting the periphery of the large diaphragm 18 and an inner ring 62 supported from the outer ring through a plurality of circumfcrentially spaced ribs 63. The inner ring 62 supports electro-mechanical transducer means for driving the diaphragms 18 and 61. This transducer herein includes a cup 64 of magnetic material supported from the ring 62 through a washer 65, also of magnetic material, a cylindrical permanent magnet 66 supported concentrically within the cup 64, a washer 67 abutting the magnet 66 and disposed concentrically in radially spaced aligned relation within the washer 65, and a cylindrical voice coil 68 disposed between the washers 65 and 67 and aixed to the diaphragms 18 and 61 at the junction therebetween. Electrical excitation of the voice coil will effect mechanical excitation of the diaphragms. If desired, a flexible centering ring 69 may support the diaphragms from an annulus 7|] secured within the frame ring 62. In accordance with the specific feature of this invention, the cup 64 is spaced from the baflle 23 and a cylindrical opening 71 is provided through the cup 64 and permanent magnet 66 to provide a passage communieating between the space directly behind the small diaphragm 61 and the space behind the speaker cup 64 which latter space is in communication with the chamber behind the speaker and the acoustical connector of the sound generator 10.

In operation, the large diaphragm 18 will have a piston-like action and will transmit sound into the space therebehind at frequencies up to a certain frequency. In this range of frequencies, the opening 71 efficiently couples the small diaphragm 61 to the space behind the large diaphragm since at low frequencies the impedance of the small passageway is not great. At about the aforementioned certain frequency, however, the impedance of the opening 71 will become considerable and no appreciable sound will be transmitted therethrough. At higher frequencies, then, the space behind the small diaphragm acts as an acoustical non-resonant compliance and this space will act as such to the high end of the audible range. Above the aforementioned certain frequency, the pistondike action of the large diaphragm no longer takes place and it is not effective to transmit sound, the small diaphragm 61, only, being effective.

Another way of eliminating transmission of sound into the space behind the speaker diaphragm at higher frequencies where the space no longer acts as a non-resonant compliance is by a network such as illustrated in Figure which has input terminals 72 and 73 and output terminals 74 and 75, with the terminals 73 and 75 connected logether and with an inductance 76 connected between the terminals 72 and 74, a capacitor 77 being connected across the output terminals 74 and 75. As the frequency increases, the inductive rcactance of the inductance 76 will increase while the capacitive reactance of the capacitor 77 will decrease so that above a certain frequency there will be no appreciable transmission of energy from the input terminals 72 and 73 to the output terminals 74 and 75. This network may, accordingly, be used to excite the speaker so as to eliminate transmission of sounds at higher frequencies where the space behind the speaker no longer acts as a non-resonant acoustical compliance.

Figure 6 is a cross-sectional view through a sound generating enclosure 78 forming another preferred embodiment of the invention. The sound generator 78 comprises a box-like rectangular cabinet having a back 79, a top 80, a bottom 81, a pair of sides 82 (only one being illustrated) and a front defined in part by a wall 83 extending downwardly from the top to a lower edge 84 spaced upwardly from the bottom 8l and by upper and lower boards 85 and 86 secured against the front edges of the top 80 and the bottom 81, respectively, and side boards 87 secured against the front edges of the sides 82.

The front wall 83 has an opening 88 therein with a speaker 89 mounted behind the opening 88. This speaker 89 comprises a frame having a large outer ring 90 secured to the front cabinet wall 83 and an inner ring 91 supported from the outer ring 90 through a plurality of peripherally spaced ribs 92, the inner ring 91 supporting a cup 93 which houses an electro-mechanical transducer. A diaphragm 94 of generally conical shape has an outer periphery secured to the outer ring and is actuated by the electro-mechanical transducer means in the cup 93.

As in the case of the sound generator 10 above described, the sound generator 78 has an acoustical connector between the back face of the speaker diaphragm 94 and the enclosed space in which the generator operates. This connector has a mouth defined by the upper edge of the lower front board 86, the inner edges of the side boards 87 and the lower edge 84 of the front cabi' net wall 83 and has a cross-sectional area continuously decreasing from the mouth to a throat coupled to the speaker diaphragm 94. For this purpose, a bafiie 95 is disposed between the sides 82 and extends rearwardly from the lower edge 84 of the front wall 83 part way toward the back 79. A second baffle 96 is also disposed between the sides 82 and extends upwardly from the rear edge of the first baffle 95 to an upper edge 97 spaced from the top 80. A third baffle 98 is disposed between the sides 82 and extends downwardly from the top 80 in the space between the bafiie 96 and the front wall 83 to a lower edge 99 spaced upwardly from the first baffle 95. The lower edge 99 of the baliie 98 cooperates with a point 100 on the bafiie` 96 to define a first throat 101 and this lower edge 99 also cooperates with the baie 95 to define a second throat 102.

The baflies 95, 96 and 98 are so disposed and so inclined relative to the back 79 and the bottom 81 as to provide a sound path of continuously increasing area from the throat 101 to the mouth of the connector at the lower front of the cabinet. This continual increase in cross-sectional area is preferably an exponential function of the distance from the throat 101. For this purpose, and also to prevent the possibility of reverberaiions between opposing parallel surfaces within the connector, a curved generally semi-cylindrical baie 103 is affixed between the back 79 and the baffle 98 about the upper edge 97 of the baie 96 and ano-ther curved bafiie 104 is disposed across the corner defined by the bottom Si and the back 79. A molding 105 having a concave upper surface may be disposed behind the lower front board 86 to prevent an abrupt change in cross-sectional area at the mouth of the connector.

The baffle 98 cooperates with the front cabinet wall 83 to define a chamber serving the dual function of coupling the diaphragm 94 to the acoustical connector at low frequencies and acting as a non-resonant compliance to the highest sound frequency transmitted by the diaphragm 94. The mean distance between the baie 98 and the front cabinet wail 83 should be not greater than one-half wave-length at the highest frequency transmitted by the diaphragm 94 and is preferably one-quarter wavelength or less.

It is essential that there be a smooth transition be tween the low frequency range where the acoustical connector is effective and the higher frequency range where sound is not transmitted through the acoustical connector and the chamber behind the diaphragm acts only as a non-resonant compliance. In some cases, as with the sound generator 10 above described, such a smooth transition is obtained with the throat of the connector directly coupled into the chamber behind the diaphragm. In other cases, however, an intermediate connector greatly improves the transition between the two ranges. In the sound generator 78, an improved transition is achieved by the provision of a short high rate of are connector between the outlet from the chamber at the throat 102 to the throat 101 of the main acoustical connector. This is herein provided by means of a molding 106 which has a concave upper surface and is affixed in the corner between the bame 95 and the baliie 96. This has the effect of increasing the impedance of the coupling between the chamber and the connector and decreasing the frequency at which the transition, or cross-over, occurs.

If desired, the speaker 89 may be constructed in a manner similar to the speaker 17 above described or, in the alternative, a network such as illustrated in Figure 5 may be used to prevent radiation of sound into the chamber at higher frequencies where the chamber no longer acts as a compliance.

It will be understood, of course, that any desired means may be provided to prevent such radiation.

Figure 7 is a cross-sectional view through a sound generating enclosure 107 forming another preferred embodiment of the invention. The enclosure 107 comprises a box-like rectangular cabinet having a back 108, a top 109, a bottom 110, a pair of sides 111 (only one being illustrated) and a front defined, in part, by a wall 112 extending downwardly from the front edge of the top 109 to a lower edge 113 spaced upwardly from the bottom and defining therewith and with the sides 111 a mouth of an acoustical connector. The connector is further defined by a baflie 114 extending rearwardly from the lower edge 113 of the front cabinet wall 112 part way toward the back 108 and a baffle 115 extending upwardly from the rear edge of the baffle 114 to an upper edge 116 spaced from the cabinet top 109 to define a throat 117 therewith. The balies 114 and 115 are inclined relative to the bottom 110 and back 108, respectively, so as to define a sound path of continuously increasing cross-sectional area from the throat 117 to the mouth of the acoustical connector. This path may be made to more nearly conform to a truly exponentially increasing path, and the possibility of reverberations between opposing parallel surfaces within the generator may be minimized by the provision of a baftie 118 across the inside corner between the back 108 and the top 109 and a baille 119 across the corner between the back 108 and the bottom 110.

The baffle 115 cooperates with the front wall 112 to define a chamber in which a speaker 120 is disposed, the speaker being secured to the front wall 112 behind an opening 121 therein. This speaker comprises a frame having an outer ring 122 secured to the front wall 112 about the opening, an inner ring 123 supported from the ring 122 through a plurality of peripherally spaced ribs 124 and supporting a cup 125 which may house an electro-mechanical transducer. A diaphragm 126 has a. periphery secured to the ring 122 and is actuated by the electro-mechanical transducer in the housing 125.

As with the sound generators and 78 above described, the chamber behind the speaker should be a nonresonant compliance to the highest frequency at which sound is transmitted from the diaphragm 126. For this reason, the mean distance between the bafiie 115 and the front cabinet wall 112 should be not greater than one-half wavelength at that frequency and is preferably less.

It will be noted that in each of the sound generators 10, 78 and 107 the acoustical connector is so formed that the baiiies are secured between the sides so as to provide a very rigid construction. This has been found to be highly important acoustically since it minimizes the possibility of resonances. All of the enclosures of the prior art of which I am aware, on the other hand, have had mechanical resonances in the audible range, usually at the low frequency end of the range so that if the enclosure is struck sharply, a boom Will be heard.

Further rigidity may be obtained in the enclosure of Figure 7 by providing an integral portion 127 on the bale 115 projecting upwardly from the central portion of the upper edge 116 thereof and locked in a recess in the top 109. This feature might if desired, be applied as well to the edge 99 of baffle 98 in the enclosure 78.

Another feature of the invention applicable to all three of the sound generators 10, 78 and 107 and having general application as well, is in the matching of the speaker relative to the acoustical connector. By this feature, the impedance of the connector is measured in free air, or in a free-field sound room, and it will be found that the connector will have a resonant peak at a frequency somewhat higher than the fiare cut-off frequency. It has been found that by making the resonant frequency of the speaker in free air equal to the frequency of the first resonant peak of the connector, a very smooth, uniform, non-resonant output is achieved from the lowest frequency of the range of frequencies generated.

It may be noted that each of the enclosures 10, 78 and 107 above described is constructed in a manner such as to eliminate the possibility of any resonances or reverberations within the frequency range transmitted and no sound absorbing materials or devices are necessary. In fact, it is highly desirable to eliminate insofar as possible any absorption of sound within the enclosures and, for this purpose, all of the internal surfaces preferably have a very hard finish. It will be understood, of course, that while sound absorption is preferably eliminated, sound absorbing material or devices may be disposed within the enclosures without departing from highly important features of the invention.

It will be further understood that each of the sound generators 10, 78 and 107 may be disposed in any desired position relative to the horizontal and that the terms back, top, sides, front, bottom, upwardly, etc. are used only for the purpose of ease of description and clarityV and are not to be construed as limitations.

It will be understood, also, that other modifications and variations may be effected without departing from the spirit and scope of the novel concepts of the present invention.

I claim as my invention:

1. In a sound generator, a box-like rectangular cabinet including a pair of sides, a back, a top, a bottom, and a front having a generally rectangular opening between said sides and extending upwardly from said bottom and having a speaker opening spaced upwardly from said rectangular opening; a first batiie disposed between said sides and extending from the upper end of said rectangular opening part way toward said back; a second baflie disposed between said sides and extending upwardly from the rear end of said first baffle and having an upper edge spaced from said top and a third batiie disposed in front of said second bafiie and extending downwardly from said top and having a lower edge spaced from said first baflie and defining therewith a throat coupled to the rear of said speaker opening.

2. In a sound generator, a box-like rectangular cabinet including a pair of sides, a back, a top, a bottom, and a front having a generally rectangular opening between said sides and extending upwardly from said bottom and having a speaker opening spaced upwardly from said rectangular opening; a first bafile disposed between said sides and extending from the upper end of said rectangular opening part way toward said back; a second bafiie disposed between said sides and extending upwardly from the rear end of said first baie and having an upper edge spaced from said top and a third baffle disposed in front of said second baie and extending downwardly from said top and having a lower edge spaced from said first baffle and defining therewith a throat coupled to the rear of said speaker opening said baffles being so disposed and inclined as to provide a sound path from said throat to said rectangular opening of continuously increasing cross-sectional area.

3. In a sound generator, a box-like rectangular cabinet including a pair of sides, a back, a top, a bottom, and a front having a generally rectangular opening between said sides and extending upwardly from said bottom and having a speaker opening spaced upwardly from said rectangular opening; a first baliie disposed between said sides and extending from the upper end of said rectangular opening part way toward said back; a second baflie disposed between said sides and extending upwardly from the rear end of said first baiiie and having an upper edge spaced from said top and defining therewith a throat coupled to the rear of said speaker opening; and a third bafiie spaced between said second bafiie and said front and having a lower edge spaced from said first baie.

4. In a sound generator, a box-like rectangular cabinet including a pair of sides, a back, a top, a bottom, and a. front having a generally rectangular opening between said sides and extending upwardly from said bottom and having a speaker opening spaced upwardly from said rectangular opening; a first baflie disposed between said sides and extending from the upper end of said rectangular opening part way toward said back; a second baiiie disposed between said sides and extending upwardly from the rear end of said first bafiie and having an upper edge spaced from said top and a third bafiie spaced behind said front for defining a speaker chamber and having a lower edge spaced from said first batiie and defining therewith a throat, said baliies being so disposed and inclined as to provide a sound path from said throat to said rectangular opening of continuously increasing cross-sectional area.

5. In a sound generator, a large peripherally supported diaphragm having a central opening therein, a small diaphragm afixed within and closing said opening, actu- `means acting at the junction between said diaphragms, means defining a relatively large chamber behind said diaphragms, and presenting an acoustical compliance to said large diaphragm up to a predetermined frequency, and means defining a small chamber behind only said small diaphragm with an elongated passage communicating between said chambers for coupling said small diaphragm to said large chamber at frequencies up to said predetermined frequency and for preventing transmission of sound from said small diaphragm to said large chamber at frequencies above said predetermined frequency.

6. In a sound generator for generating sound in a low frequency range extending up to a predetermined frequency, a cabinet having a speaker opening and a second opening of substantially greater area than said speaker opening, a speaker having a diaphragm closing said speaker opening, iirst baflie means spaced behind said diaphragm a distance on the order of one-fourth wave length or less at said predetermined frequency to define a chamber which presents a non-resonant compliance up to said predetermined frequency, and additional baflle means disposed within said cabinet and cooperating with the cabinet walls to provide a sound path of increasing cross-sectional area having a mouth at said second opening and a throat coupled to said chamber, said throat having an area of at least one-eighth the area of said mouth and small enough to highly attenuate transmission of sound from said chamber into said sound path at frequencies above a cross-over frequency on the order of 150 cycles per second, said sound path having a free-air flare cut-oi frequency of less than 60 cycles per second, and said predetermined frequency being substantially greater than said cross-over frequency.

7. A sound generator as defined in claim 6 in which said cabinet includes a pair of sides, a back, a top, a bottom, and a front having said second opening therein between said sides and extending upwardly from said bottom and having said speaker opening therein at a position spaced upwardly from said second opening, and in which said first and additional baiiie means include one baffle disposed between said sides and extending from the upper end of said second opening part way toward said back and another bafe extending upwardly from the rear end of said one baille behind said diaphlagm.

References Cited in the le of this patent UNITED STATES PATENTS 1,869,178 Thuras July 26, 1932 2,205,804 Wells lune 25, 1940 2,224,919 Olson Dec. 17, 1940 2,310,243 Klipsch Feb. 9, 1943 2,534,040 Lindley Dec. 12, 1950 2,568,883 Carrington Sept. 25, 1951 2,604,182 Massa July 22, 1952 2,643,728 Anthony June 30, 1953 FOREIGN PATENTS 337,264 Great Britain Oct. 30. 1930 908,709 France Apr. 17, 1946 OTHER REFERENCES Johnson: publication, Radio and Television News,

September 1951, pages 63 and 144.

Text book, Elements of Acoustical Engineering, by Olson., pages 13S-140.

Publication, Audio Engineering, July 1952, pages 26, 27, 28 and 34. 

